1. Field of the Invention
The present invention relates generally to audio codecs and specifically with the tuning of high-pass filters within audio codecs.
2. Related Art
Excessive low frequency (“bass”) sounds are often problematic for personal computers (PC). Both internal and external speakers on PCs are typically small and can be damaged if too much power is delivered at low frequencies. Excessive low frequency sound may also cause problems with components within a personal computer. For example, the vibrations from low frequency sound can lead to data corruption and even failure of a hard drive device. FIG. 1 shows a prior art system which attenuates harmful low frequency signals. The audio signal is supplied by HD audio driver 102, which is filtered by high-pass filter 104 and drives speaker 106.
The advent of high definition (HD) audio standards in PCs has imposed certain audio performance requirement on external outputs, such as headphones or microphone jacks. However, such requirements are not imposed on integrated speakers which allows a high-pass filter to be inserted into the audio path to integrated speakers. The HD-audio standards also allow for individual digital to audio converters (DACs) to be assigned to arbitrary audio paths. Under the standard, the HD codec must operate with a generic audio driver. Therefore, if a high-pass filter is included in the audio path, specialized driver software would not be available to enable or adjust it and the high-pass filter must be automatically tuned or set in the factory by a vendor.
These limitations are not an obstacle to HD audio codecs where low impedance drivers, such as speaker drivers, are separate from the audio codec. A user or vendor could place a high-pass filter between the output of the codec and the speaker driver, leaving all other audio paths unfiltered.
FIG. 2 illustrates a HD audio system as known in the art. Integrated circuit 210 comprises DAC 202 and line driver 206 for a speaker path and DAC 204 and headphone driver 208 for the headphone path. DAC 202 and DAC 204 receive audio input from HD audio interface 260 as digital data. DAC 202 and DAC 204 each convert their respective digital audio signals into an analog audio signal. The analog audio signal passed to line driver 206 is amplified and provided to line output 252 which can be connected to a speaker driver. The analog audio signal passed to the headphone driver is provided to headphone output 254 which may be connected to a jack, where headphone 244 could be attached. The line out analog audio signal does not have sufficient power to drive a speaker, so speaker driver 232 is employed to amplify the audio signal to drive speaker 242 in a low impedance interface. In this system, speaker driver 232 is provided on integrated circuit 230 which is separate from integrated circuit 210. In order to prevent damage to speaker 242, high-pass filter 220 is inserted into the audio path between line driver 206 and the speaker driver 232. This high-pass filter can be a resistor 214 connected to ground and capacitor 212 in series with the audio path as shown in the figure.
Because the desired frequency response of the high-pass filter is often based on the speaker characteristics, the speaker manufacturer either supplies or specifies requirements for the high-pass filter. In the architecture shown in FIG. 2, neither the manufacturer of integrated circuit 210 nor the manufacturer of integrated circuit 230 needs to know the requirements of the high-pass filters.
In today's trend of further integration, integrated circuits, audio codecs are being integrated with the speaker drivers on the same chip. This complicates access to the audio path for the insertion of a high-pass filter. Therefore, there is a need in the industry to insert a high-pass filter into an HD audio path while meeting HD audio requirements including compatibility with a generic software driver, that is, without the ability to tune the high-pass filter through software.